Global correlations of ocean ridge basalt chemistry with axial depth: A new perspective

Abstract

The petrological parameters Na8 and Fe8, which are Na2O and FeO contents in mid-ocean ridge basalt (MORB) melts corrected for fractionation effects to MgO = 8wt%, have been widely used as indicators of the extent and pressure of mantle melting beneath ocean ridges. We find that these parameters are unreliable. Fe8 is used to compute the mantle solidus depth (Po) and temperature (To), and it is the values and range of Fe8 that have led to the notion that mantle potential temperature variation of δTP = 250 K is required to explain the global ocean ridge systematics. This interpreted δTP = 250 K range applies to ocean ridges away from 'hotspots'. We find no convincing evidence that calculated values for Po, To, and δTP using Fe8 have any significance. We correct for fractionation effect to Mg# = 72, which reveals mostly signals of mantle processes because melts with Mg# = 0.72 are in equilibrium with mantle olivine of Fo89-6 (vs evolved olivine of Fo88-79-6 in equilibrium with melts of Fe8). To reveal first-order MORB chemical systematics as a function of ridge axial depth, we average out possible effects of spreading rate variation, local-scale mantle source heterogeneity, melting region geometry variation, and dynamic topography on regional and segment scales by using actual sample depths, regardless of geographical location, within each of 22 ridge depth intervals of 250 m on a global scale. These depth-interval averages give Fe72 = 7·5-8·5 which would give δTP = 41 K (vs∼250 K based on Fe8) beneath global ocean ridges. The lack of Fe72-Si72 and Si72-ridge depth correlations provides no evidence that MORB melts preserve pressure signatures as a function of ridge axial depth. We thus find no convincing evidence for δTP≥50K beneath global ocean ridges. The averages have also revealed significant correlations of MORB chemistry (e.g. Ti72, Al72, Fe72, Mg72, Ca72, Na72 and Ca72/ Al72) with ridge axial depth. The chemistry - depth correlation points to an intrinsic link between the two. That is, the ∼5 km global ridge axial relief and MORB chemistry both result from a common cause: subsolidus mantle compositional variation (vs δTP), which determines the mineralogy, lithology and density variations that (1) isostatically compensate the ∼5 km ocean ridge relief and (2) determine the first-order MORB compositional variation on a global scale. A progressively more enriched (or less depleted) fertile peridotite source (i.e. high Al2 O3 and Na2O, and low CaO/Al2 O3) beneath deep ridges ensures a greater amount of modal garnet (high Al2O3) and higher jadeite/diopside ratios in clinopyroxene (high Na2O and Al2 O3, and lower CaO), making a denser mantle, and thus deeper ridges. The dense fertile mantle beneath deep ridges retards the rate and restricts the amplitude of the upwelling, reduces the rate and extent of decompression melting, gives way to conductive cooling to a deep level, forces melting to stop at such a deep level, leads to a short melting column, and thus produces less melt and probably a thin magmatic crust relative to the less dense (more refractory) fertile mantle beneath shallow ridges. Compositions of primitive MORB melts result from the combination of two different, but genetically related processes: (1) mantle source inheritance and (2) melting process enhancement. The subsolidus mantle compositional variation needed to explain MORB chemistry and ridge axial depth variation requires a deep isostatic compensation depth, probably in the transition zone. Therefore, although ocean ridges are of shallow origin, their working is largely controlled by deep processes as well as the effect of plate spreading rate variation at shallow levels. © The Author 2007. Published by Oxford University Press. All rights reserved.